Re: [PATCH v4] staging: greybus: Convert uart.c from IDR to XArray

["Fabio M. De Francesco" <fmdefrancesco@gmail.com>]

On Monday, August 30, 2021 11:12:28 AM CEST Johan Hovold wrote:
> On Sun, Aug 29, 2021 at 11:22:50AM +0200, Fabio M. De Francesco wrote:
> > Convert greybus/uart.c from IDR to XArray. The abstract data type XArray
> > is more memory-efficient, parallelisable, and cache friendly. It takes
> > advantage of RCU to perform lookups without locking. Furthermore, IDR is
> > deprecated because XArray has a better (cleaner and more consistent) API.
> 
> Where does it say that IDR is deprecated? Almost all drivers use IDR/IDA
> and its interfaces are straight-forward. In most cases we don't care
> about a possible slight increase in efficiency either, and so also in
> this case. Correctness is what matters and doing these conversions risks
> introducing regressions.
> 
> And I believe IDR use XArray internally these days anyway.

Please read the following message by Matthew Wilcox for an authoritative response to your
doubts about efficiency of XArray and IDR deprecation:
https://lore.kernel.org/lkml/20210503182629.GE1847222@casper.infradead.org/

In particular he says that "[] The advantage of the XArray over the IDR is that it has a better 
API (and the IDR interface is deprecated).".

> > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
> > ---
> > 
> > v3->v4: 
> > 	Remove mutex_lock/unlock around xa_load(). These locks seem to
> > 	be unnecessary because there is a 1:1 correspondence between
> > 	a specific minor and its gb_tty and there is no reference
> > 	counting. I think that the RCU locks used inside xa_load()
> > 	are sufficient to protect this API from returning an invalid
> > 	gb_tty in case of concurrent access. Some more considerations 
> > 	on this topic are in the following message to linux-kernel list:
> > 	https://lore.kernel.org/lkml/3554184.2JXonMZcNW@localhost.localdomain/
> 
> This just doesn't make sense (and a valid motivation would need to go in
> the commit message if there was one).

OK, I'll take your words on it. Alex Elder wrote that he guess the mutex_lock/unlock()
around xa_load() are probably not necessary. As I said I don't yet have knowledge of 
this kind of topics, so I was just attempting to find out a reason why. Now I know that 
my v1 was correct in having those Mutexes as the original code with IDR had.
>
> > [...]
> 
> You can't just drop the locking here since you'd introduce a potential
> use-after-free in case gb_tty is freed after the lookup but before the
> port reference is taken.
> 
> That said, this driver is already broken since it can currently free the
> gb_tty while there are references to the port. I'll try to fix it up...
> 
> >  	return gb_tty;
> >  }
> 
> But as you may have gathered, I don't think doing these conversions is a
> good idea.
> 
> Johan
> 

Thanks for your review,

Fabio